In recent years, electronic apparatuses have been downsized, and they use large amounts of electric current. This market trend requires inductance components employed in these electronic apparatuses to be downsized, and yet to be driven by large amounts of electric current.
An inductance component is chiefly formed of a coil and a magnetic material inserted in the coil. The magnetic material used in the inductance component falls into two main groups, namely, a ferrite core and a powder magnetic core as a composite magnetic body. Since the ferrite core has so small saturation magnetization, it tends to be encountered by magnetic saturation. In the presence of large amounts of electric current needed for the recent electronic apparatuses, a magnetic permeability of the ferrite core thus remarkably decreases. Some measures are taken against this problem: a cross section through which the magnetic flux of the ferrite core travels is enlarged; or a gap is disposed in the ferrite core to suppress the magnetic saturation to occur. However, the enlargement of the cross section causes the inductance component to be bulky, and the employment of the gap causes a leakage flux that invites an increase of eddy current loss at the coil, and also produces noises to peripheral components. It is thus difficult for the existing techniques to achieve a ferrite core that is downsized and driven by a large amount of electric current.
On the other hand, the powder magnetic core produced by compressing and molding the metal magnetic powder has a greater saturation magnetization, so that its magnetic permeability decreases by a smaller amount than the ferrite core in the situation of using the large amount of electric current. The powder magnetic core is thus useful for manufacturing the inductance components that can be driven by large amounts of electric current and downsized.
The powder magnetic core needs a certain mechanical strength in order to suppress cracks or breakages in manufacturing or in use, and to increase the yield or to enhance the reliability.
To increase a mechanical strength of the composite magnetic body, a filling factor of metal magnetic particles that form metal magnetic powder is conventionally increased, so that mechanical entanglement among the metal magnetic particles is facilitated and the mechanical strength of the powder magnetic core is increased. However, the increase in the filling factor of metal magnetic particles is not enough to obtain sufficient mechanical strength, so that it is difficult for this method to produce the powder magnetic core that has excellent magnetic properties as well as sufficient mechanical strength. Therefore, use of impregnation processing for improving the mechanical strength has been studied, and then a powder magnetic core of which mechanical strength and magnetic properties are improved by this method is disclosed. Related art to this method is disclosed in, for example, Patent Literatures 1 to 3.
Patent Literature 1 discloses the following method: metal magnetic powder is mixed with first binder as molding assistant agent to produce granulated powder, which is then pressurized and molded to produce a compact. This compact undergoes a thermal treatment, and then is impregnated with a second binder. The process discussed above allows increasing the mechanical strength.
Patent Literature 2 discloses that a polymeric resin is impregnated into at least a part of voids in a composite magnetic body, thereby increasing the mechanical strength.
Patent Literature 3 discloses that use of methacrylic acid diester as an impregnating resin allows suppressing the magnetic properties of the powder magnetic core to be lowered while the mechanical strength is effectively increased.